The piggyback cable is intentionally connected to ground at its remote end, a fact that makes conventional detection of faults in that region extremely difficult. The piggyback cable has a linearly decreasing voltage, from an input value at its power in-feed end to zero at the grounded, remote end (see FIG. 1). Consequently, the electric field stress on the cable insulation also decreases linearly, from a normal operating stress at the power in-feed end to zero at the remote end.
A cable fault in the remote region may be initiated by a mechanical damage, e.g. a cut extending through the outer sheath and the insulation system, thus exposing the copper conductor to seawater. As the conductor is connected to ground at the remote end, the fault will shunt its remaining length from fault location to grounded end. The corresponding change in conductor current will be minute and extremely difficult to detect at the opposite end of the piggyback cable. In most practical installations, the current measurement will be done even further upstream, making small changes even harder to detect. The conductor current in a DEH system is typically larger than 1,000 A, and a fault current of 10 A (through the physical fault) will translate into a far smaller change at the in-feed end (due to phase shifting). Even with the best available current measuring equipment, cable faults near the remote end will therefore pass on undetected.
An electric current flowing out from the surface of a copper conductor and into seawater will cause rapid (a.c.) corrosion of the copper conductor, even at small current levels or voltage differences. If such a fault goes undetected, the final outcome will be a complete corrosion break of the copper conductor. A seawater filled gap is thus introduced between the two “conductor stubs”, but the electric impedance of this gap may not be sufficiently large to cause a detectable change in current at the in-feed end of the DEH system. As the gap will not be capable of withstanding the source voltage, an electric arc is then formed between the two “conductor stubs”. The temperature associated with such arcing is several thousand degrees Celsius, so a rapid meltdown of the copper conductor as well as any polymer in the vicinity will occur. The boiling temperature of seawater at most relevant water depths will be above the polymer melting points, so “water cooling” will not prevent the described melt-down from taking place.
The piggyback cable is commonly placed as close to the thermally insulated pipeline as possible, as this yields optimum DEH system efficiency. The pipeline's thermal insulation will thus also be melted down by a fault as described above. Once the steel pipeline is exposed to seawater it will appear as an alternative, and probably low-impedance, return path for the fault current. As the copper conductor is continuously eroded away and widening the gap between the “stubs”, the pipeline will at some point in time become the lowest impedance return path. At that time, a new arc will be established between the conductor stub (in-feed side) and the steel pipeline. A rapid melt through of the pipeline's steel wall will result, and the pipeline contents will escape. Consequences of this may be very serious.
There is still no large change in the DEH in-feed current, and thus no indication of fault. A drop in pipeline pressure will be the first indication that something is wrong, but at that time the pipeline has already been ruptured.
The challenge before the present inventor thus was to establish an alternative DEH cable fault detection principle which will provide a clear fault indication before the pipeline may be damaged.
Conventional systems for protection of DEH cables comprise impedance protection and differential current protection. Both protection systems work by measuring electrical quantities at the in-feed end of the DEH system. Differential protection also requires measured current at the remote (sub sea) end.
Impedance protection is based on measuring the in-feed voltage in addition to the in-feed current, and thus becomes relatively robust with respect to voltage variations. However, it is considered that this method principally must leave a larger “blind zone” at the end or remote region discussed above, than the differential current protection.
Differential current protection is regarded as being the more robust, but the practical implementation is both costly and complicated. A current sensor is required at the remote end, together with communication back to the in-feed end, power supply etc. Further, it is principally impossible to completely eliminate the “blind zone” near the grounded end, as any practical measuring system will have a limited accuracy.
Detection of cable damage through the use of optical fibers is described in U.S. Pat. No. 6,559,437. The main objective of that patent is to provide a means of detecting mechanical damage to the cable insulation (e.g. by fraying, cutting or abrasion). Detection is achieved by placing an optical fiber sensor within the cable insulation and monitoring the condition of this sensor. The resulting fault detection system is said to be capable of detecting a fault manifested by “cutting or severing the fiber optic sensor, excessive pressure on the fiber optic sensor, or unduly high temperatures”.
Optical fiber break detection could provide a means of detecting cable break within the remote end region. However, practical integration of a fiber optic sensor as described by U.S. Pat. No. 6,559,437 into the physically large DEH piggyback cable is considered non-feasible from a mechanical point of view. I.e. the practical handling of a fiber optic sensor during cable manufacturing, handling and installation would not be consistent with the capabilities of the optical fiber.
Commercial products exist for obtaining a temperature profile along an optical fiber. Roughly, the alternatives may be divided into two main categories: a) measurement of temperature at predefined sensor locations (e.g. Bragg gratings), and b) distributed measurements on a homogenous optical fiber. In principle, a local temperature increase at any location along the fiber may be detected by a category b) monitoring system. However, difficulties in fault detection on such basis will be serious, in particular at large DEH cable lengths.
The practical integration of an optical fiber element into a single core cable is described in EP0825465 (belonging to the present applicant). A number of optical fibers are enclosed by a common metal tube for the purpose of avoiding direct handling of the optical fibers during integration into the much larger single core cable. The fiber/tube element is oscillated (preformed) prior to insertion into the single core cable, making it capable of enduring the mechanical stress introduced by normal bending and handling of the cable. This EP patent specification does not touch upon the problem of fault detection in DEH cables.
Other examples of composite power cables with fiber optic elements for communication purposes are found in EP0539915 and EP0603604.